The present invention relates to a power transistor of high breakdown voltage in, for example, multi-emitter or multi-base structure. More particularly, it relates to a power transistor capable of switching at high speed.
Hitherto, in the power bipolar transistor for switching power source for handling a large electric power, higher breakdown voltage, larger current, faster switching, and wide safe operating area (SOA) are being demanded. Accordingly, it has been generally attempted to heighten the resistance by increasing the specific resistance or thickness of the collector layer. For higher breakdown voltage, moreover, it is known effective to increase the thickness of the base region because the electric field concentration can be alleviated in the curvature part of pn junction.
Moreover, as shown in FIG. 8, it is also known to enhance the breakdown voltage by using a diffusion wafer of high resistance, or using field limiting ring (FLR)44 or field plate (FP)45. More specifically, in the planar transistor of high breakdown voltage, theoretical breakdown voltage is not obtained because of impurities such as electric charge at the interface of the surface of semiconductor layer and insulating film such as electric charge at the interface of the surface of semiconductor layer and insulating film such as oxide film. For that reason, a wafer of higher specific resistance than theoretical value is used, and a field limiting ring 44 is provided to expand the depletion layer of the base-collector junction to the outer periphery of the collector region to enhance the breakdown voltage (for example, see the Japanese Unexamined Patent Publication No. 135158/1986), or the negative bias side electrode is extended to cover the surface of pn junction to compensate the positive electric charge in the oxide film of the planar junction by electric field, and a field plate 45 with a stable surface state is provided, thereby enhancing the breakdown voltage.
This conventional transistor comprises a collector region 41 made of a semiconductor layer 41b of n.sup.- type low impurity concentration formed by epitaxial growth on a semiconductor substrate 41a of n.sup.+ type, a p type base region 42 formed in the collector region 41 by diffusion or the like, an n.sup.+ type emitter region 43 formed in the base 42 by diffusion or the like, and a field limiting ring 44 of p type, the same conductive type as the base, is provided outside the base-collector junction so as to surround the base-collector junction. A field plate 45 is formed as the end portion of the base electrode is provided over the pn junction. Reference numeral 46 is an annular region of an element boundary.
The constitution is the same in the multi-type transistor of multi-emitter or multi-base structure. The operating current of the transistor requiring a large output, that is, the collector current of the absolute maximum rating is mainly related with the area and peripheral length of the emitter, and to increase this operating current, it is needed to increase the emitter area and peripheral length so as to lower the current density. Hence, a transistor of multi-emitter or multi-base structure is considered.
To maintain a high frequency characteristic, moreover, it is necessary to decrease the thickness of the base region so as to decrease the junction capacity and of the base region shorten the time .tau..sub.b for the carrier to run through the base region. On the other hand, to enhance the breakdown voltage, it is necessary to increase the thickness of the base region as mentioned above. However, when the thickness of the base region is increased, the high frequency characteristic is imparied, and the switching speed in the proportional relation becomes slow. Hence, to maintain high frequency characteristic and high breakdown voltage, for example, a transistor of comb-shaped emitter structure as shown in FIG. 9 is proposed by Tsutomu Nozaki in "Searching the latest trend of transistor" (Transistor Technology published by CQ Publishing Co., January 1993, pp. 300-301).
In the transistor of comb-shaped emitter structure shown in FIG. 9, for example, on a collector region 56 of n conductive type, a base region 54 of p conductive type, an emitter region 57a of n conductive type, and an insulating film 58 are formed, and multiple comb-shaped emitter regions 57 a are formed in the base region 54, and it is composed so that the base region 54 of the bottom side of the emitter region 57a may be thinner. Openings are provided in insulating films 58 on the emitter region 57 and base region 54, and an emitter contact 52 and a base contact 53 are formed, and the emitter electrode wiring 51 and base electrode wiring 55 by coupling them are respectively connected to the emitter pad 49 and base pad 50. As a result, the emitter region 57a is formed in a thin comb shape, and the base region 54 beneath the emitter region 57a is thin, and hence the high frequency characteristic is improved, and the surrounding base region is formed thick, so that a higher breakdown voltage is achieved.
On the other hand, in order to shorten the storage time which is important as switching-off characteristic (the time returning to the active region by releasing excess carrier from the saturated state accumulating excess carrier in the base and collector region), for example, as shown in lines 2 to 7 of lower left column of page 2 and FIGS. 1 and 2 of the Japanese Unexamined Patent Publication No. 160165/1986, a bipolar transistor capable of controlling the storage time efficiently is proposed. This bipolar transistor is formed by connecting an internal base layer 64 stradding over the collector layer 62 and collector epitaxial layer 63 on the semiconductor substrate 61 to the base layer 65. In such bipolar transistor incorporating the internal base layer 64, the carrier injected from the emitter 66 easily recombined in the internal base layer 64, and the carrier life time can be shortened. While, paying attention to the carrier injected in the collector side, when changing over the transistor from ON state to OFF state, the carrier is absorbed by the inverse base current flowing through the internal base layer 64 of low resistance, and is recombined. In this transistor, since the carrier is absorbed through the internal base layer 64 of low resistance, the storage time can be shorter than in the prior art.
In the transistor provided with field limiting ring and field plate in order to heighten the breakdown voltage of transistor, they are ususally provided outside the base region, and the number of field limiting rings must be increased in order to have a sufficient breakdown voltage, which requires a wider collector area, resulting in a large chip size.
While, in the transistor enhanced in breakdown voltage by forming the collector region in a high resistance layer, the saturation voltage between collector and emitter V.sub.CE (sat) is high, and the effective emitter area must be widened, and hence the chip size becomes large, and the base region must be made thick in order to heighten the breakdown voltage, and hence a longer diffusion time is needed, and the cost is increased.
In the transistor as shown in FIG. 9, on the other hand, the base region is shallow, and a certain high frequency characteristic may be maintained and a high breakdown voltage can be realized, but the minority carrier which is the factor of impeding the high speed switching action of transistor cannot be extracted, and hence the time of the switching is slow.
Further, in the bipolar transistor as shown in FIG. 10, since the minority carrier can be extracted, a high speed switching action can be realized, but since the base region is shallow, the safe operating area (SOA) is narrow, and enhancement of breakdown voltage is insufficient.